Apparatuses using laser light as a light source of a projection type projector, which is used for a movie or home theater, have been developed. In such a laser light source which serves as a light source, there are case where light emitted directly from a semiconductor laser element is used, and case where the wavelength of light emitted from the semiconductor laser element is converted to wavelength different therefrom by a nonlinear optical crystal in order to use the light. In recent years, laser light sources have been developed as blue or green laser light sources in which periodically poled lithium niobate (PPLN: Periodically Poled Lithium Niobate), or periodically poled lithium tantalate (PPLT: Periodically Poled Lithium Tantalate), etc. is used for such a nonlinear optical crystal.
For example, what is disclosed in Japanese Patent Application Publication No. 2009-54446 is known as such technology. The Japanese Patent Application Publication discloses that a laser light source apparatus comprises a light source, which is made up of a semiconductor laser; a wavelength conversion element (which is a nonlinear optical crystal, for example, PPLN) which receives and converts laser light emitted from the light source, into second harmonics; and an external resonator which chooses light of predetermined wavelength emitted from the wavelength conversion element, and which reflects it toward the light source (for example, volume bragg grating: VBG: Volume Bragg Grating). Moreover, it discloses that a temperature adjusting unit is provided between the wavelength conversion element and a subbase to which the wavelength conversion element is attached. Furthermore, it discloses that since a pitch of a polarization reversal cycle of the wavelength conversion element can be adjusted by adjusting the temperature of the wavelength conversion element using the temperature adjusting unit, it becomes possible to improve the light conversion efficiency.
FIG. 14 is a block diagram showing a schematic configuration of a laser light source apparatus. A wavelength conversion element 5 (for example, PPLN) mounted on a laser light source unit LH has a function to perform wavelength conversion for converting the wavelength of light emitted from the laser light source element 2 (which is, for example, a semiconductor laser, and which will be referred to as a semiconductor laser) into wavelength shorter than that of the incidence light, wherein, for example, infrared rays can be converted into green light. A lightning circuit 20 comprises an electric power supply circuit U1 and a pulse circuits U2 which supplies pulse-like electric power, wherein voltage/current for lighting the semiconductor laser 2 is outputted. In this wavelength conversion element 5, quasi phase matching is carried out so as to raise the optical conversion efficiency, by raising it to predetermined temperature, so that very accurate temperature control is needed. Therefore, a heating unit 7 (hereafter explained as a heater 7) for heating the wavelength conversion element 5 is provided, and a temperature detection unit Th1 which detects the temperature of the heater 7, for example, a thermistor, is arranged.
Moreover, a control unit 21 comprises a control means 21a, a temperature control unit 21b, and a drive circuit U3 which drives the heater 7. The electric power supply circuit U1 performs control so that voltage impressed to the semiconductor laser 2 and current to be passed therethrough may be a preset value or a value set up from the outside, by the control means 21a of the control unit 21. Moreover, start or stop of the electric power supply etc. is controlled thereby. The control means 21a of the control unit 21 and the temperature control unit 21b are configured by an arithmetic processing unit (a CPU or a microprocessor). Moreover, the pulse circuit U2 is controlled by the control means 21a. The control means 21a turns on and off a switching element of the pulse circuit U2, and generates a pulse output which drives the semiconductor laser 2. The temperature control unit 21b controls the amount of electric power supplied to the heater 7 based on a difference between the temperature detected by the temperature detection unit Th1 and the preset temperature, which is target temperature, thereby performing feedback control so that the temperature of the wavelength conversion element may conform the setting temperature.
A control system, which is generally known as “ON/OFF-PID control” can be used” as the above-described feedback control system. The PID control is a method for performing control so that a proportionality element, an integration element, and a differentiation element are combined with one another, so as to become a target temperature. In addition, for example, a value of approximately several kilohertz is adopted as the frequency of a PWM output used in the present embodiments.
FIG. 15 is a flow chart showing an example of control and processing in the temperature control unit 21b of the control unit 21. The flow chart shown in FIG. 15 can be realized by software process in a microcomputer mounted in the above-described control unit 21. The temperature control unit 21b of the control unit 21 performs, for example, processing shown in the flow chart, so that the temperature of the wavelength conversion element 5 may be controlled so as to turn into the preset temperature. In order that the temperature control unit 21b of the control unit 21 controls the temperature of the wavelength conversion element 5 so as to turn into target temperature, the temperature detection unit Th1 detects the temperature of the wavelength conversion element 5, and an output manipulating value to the heater 7 is periodically put in execution and is controlled by comparing the detected temperature with the preset temperature which will serve as the target temperature. With respect thereto, a typical technique, which is PI control, where proportionality element and integration element are combined with each other, will be explained below as an example.
In FIG. 15, heater control starts at Step (B01). First, in Step (B02), a value of the temperature of the wavelength conversion element 5 (PPLN actually measured temperature value), is actually measured by the temperature detection unit Th1, whereby the actually measured temperature (Tm_PPLN) is obtained. Next, in Step (B03), target temperature of the wavelength conversion element 5, i.e. a temperature setting value (PPLN temperature setting value) of the wavelength conversion element 5 is read out, so that an optimal temperature setting value (Ts_PPLN) is obtained. And in Step (B04), the temperature setting value (Ts_PPLN) and the actually measured temperature value (Tm_PPLN) measured by the temperature detection unit Th1 are compared with each other, thereby a difference (en) therebetween is obtained. In Step (B05), a PI operation is performed using this difference (en). In this PI operation, the amount of electric power supplied to the heater 7, i.e., a manipulating value to the heater 7, is calculated from expression (1) shown below.MVn=MVn-1+Kp×en+Ki×en-1  (1)
Here, MVn, MVn-1, en, and en-1 respectively represent this time manipulating value, last cycle manipulating value, a temperature difference value which is calculated this time, and a temperature difference value which is calculated in last cycle, and Kp and Ki are constants.
Although the manipulating value (MVn) computed by the PI operation will be updated as an ON width of a PWM signal sent out from the control unit 21, when the manipulating value (MVn) exceeds the maximum (MVn upper limit) in Step (B06), the maximum value is treated as the manipulating value (MVn), and when it is less than the minimum (MVn lower limit) in Step (B07), the minimum value is treated as the manipulating value (MVn), whereby upper and lower limit restrictions are performed (Step (B08), and Step (B09)). And in the Steps (B06-B9), the manipulating value, which is finally determined, is updated as an ON width (Duty (n)) of the PWM signal to be sent out from the control unit 21, and the heater control of that cycle ends (Steps (B10, B11)). A series of the operations from Step (B01) to Step (B11) is repeated in a predetermined cycle. The control is stably performed so that the wavelength conversion element 5 becomes optimal in temperature, by performing this flow chart periodically so as to perform feedback control. Although the PI control method which is made up of proportional control and an integration element is used in the above-explained control algorithm, other feedback control methods including, for example, control such as the PID control where a differentiation (differentiation) element is added, may be used.
In addition, as for the wavelength conversion element 5 (for example, PPLN), the conversion efficiency of laser light changes with the temperature of the wavelength conversion element, and there is the optimal temperature at which the optical conversion efficiency can be maximized. For this reason, as for the above-mentioned temperature control unit 21b, it is common that the heater 7 is controlled so that the temperature of the wavelength conversion element 5 detected by temperature detection unit Th1 may become temperature at which the above-mentioned optical conversion efficiency is maximized, in order to control that temperature.